Il8 RITTER 



be said here. The cavity in the anterior end is unusually vol- 

 uminous, and in some places the walls become relatively thin, 

 though everywhere they show distinctly the characteristic noto- 

 chordal structure. The ventral coecum immediately in front of 

 the body of the nuchal skeleton is well developed. The isth- 

 mus is, as in all other species, much reduced, both in diam- 

 eter and thickness of wall, as compared with the portion farther 

 forward in the proboscis. It, however, maintains its size and 

 distinctness in a greater degree than is usual. A few facts in 

 connection with the portion of it immediately in front of the 

 communication with the esophagus deserves particular atten- 

 tion. A section from this locality is shown in PI. VII, fig. 5 . As 

 maybe seen from this figure, the isthmus is here divided length- 

 wise, sharply, into three portions, a median portion with a 

 well-defined lumen {^in.n.no-c), and two lateral depressed, 

 wing-like portions {l.iv. and r. w.), in which the lumen is 

 nearly obliterated. It will be noted that the wing to the left in 

 the figure is entirely separated from the median portion by a 

 lamina of the nuchal skeleton. A few sections farther back 

 the cut-off wing disappears entirely. The right wing under- 

 goes the same cutting off and disappearance, though the sep- 

 arating skeletal lamina comes in on this side a few sections be- 

 hind the one here figured. Eighteen sections still farther back 

 the body of the nuchal skeleton becomes divided lengthwise by 

 a narrow slit, /. e.^ the skeletal crura arise, and the cubical 

 celled epithelium forming the ventral wall of the notochord 

 (PI. VII, fig. 5, V. cp.) reaches down through this slit and be- 

 comes continuous with the dorsal gut epithelium, thus produc- 

 ing an appearance in section like that shown in fig. 6. This 

 lengthwise division of the isthmus is]|not a constant feature in 

 all specimens, and the cutting off and disappearance of the 

 wings I have found in the one case only. The condition is, 

 however, none the less significant from its inconstancy. 



The thickening of the dorsal esophageal wall adjacent to 

 the notochord and the vacuolation of the outer layer of its cells 

 should be noticed here (PI. VII, fig. 5). 



From the level at which the wings disappear the lateral pro- 

 jections of the isthmus, seen at .v and y (fig. 5), extend 



